Organic electroluminescent device having a conjugated polymer and an inorganic insulative electron injecting and transporting layer

ABSTRACT

An organic electroluminescent device having high efficiency, a long life time, and low cost, includes a substrate, a hole injecting electrode and an electron injecting electrode formed on the substrate, an organic material-containing organic layer between the electrodes, and an inorganic insulative electron injecting and transporting layer between the light emitting layer and the electron injecting electrode. The organic layer includes a light emitting layer containing a conjugated polymer, and the inorganic insulative electron injecting and transporting layer comprises three components. The first component is at least one oxide selected from the group consisting of lithium oxide, rubidium oxide, potassium oxide, sodium oxide, and cesium oxide. The second component is at least one of strontium oxide, magnesium oxide, and calcium oxide. The third component consists of silicon oxide, germanium oxide, or mixtures thereof.

BACKGROUND OF THE INVENTION

Technical Field

This invention relates to an organic electroluminescent (EL) device andmore particularly, to an inorganic/organic junction structure suitablefor use in a device of the type wherein an electric field is applied toa thin film of an organic compound to emit light.

Background Art

Active research works have been made on organic EL devices for use asdisplays because EL devices can be formed on a large area of glass. Ingeneral, organic EL devices have a basic configuration including a glasssubstrate, a transparent electrode of tin-doped indium oxide (ITO) etc.,a hole transporting layer of an organic amine compound, a light emittinglayer of an organic fluorescent material exhibiting electronicconductivity and intense light emission such as an aluminum quinolinolcomplex (Alq3), and an electrode of a metal having a low work functionsuch as MgAg, wherein the layers are stacked on the substrate in thedescribed order.

The device configurations which have been reported thus far have one ormore organic compound layers interposed between a hole injectingelectrode and an electron injecting electrode. Structures having two orthree organic compound layers are typical.

Included in the two-layer structure are a structure having a holetransporting layer and a light emitting layer formed between the holeinjecting electrode and the electron injecting electrode and anotherstructure having a light emitting layer and an electron transportinglayer formed between the hole injecting electrode and the electroninjecting electrode. Included in the three-layer structure is astructure having a hole transporting layer, a light emitting layer, andan electron transporting layer formed between the hole injectingelectrode and the electron injecting electrode. Also known is aone-layer structure wherein a single layer playing all the roles isformed from a polymer or a mixed system.

FIGS. 3 and 4 illustrate typical configurations of organic EL devices.

In FIG. 3, a hole transporting layer 14 and a light emitting layer 15,both of organic compounds, are formed between a hole injecting electrode12 and an electron injecting electrode 13 on a substrate 11. In thisconfiguration, the light emitting layer 15 also serves as an electrontransporting layer.

In FIG. 4, a hole transporting layer 14, a light emitting layer 15, andan electron transporting layer 16, all of organic compounds, are formedbetween a hole injecting electrode 12 and an electron injectingelectrode 13 on a substrate 11.

Reliability is a common problem to be solved for these organic ELdevices. More particularly, organic EL devices in principle have a holeinjecting electrode and an electron injecting electrode and need organiclayers for effectively injecting and transporting holes and electronsfrom the electrodes, respectively. However, the organic materials ofwhich the organic layers are formed are vulnerable during manufactureand have less affinity to the electrodes. Another problem is raised bythe significantly accelerated degradation of organic thin films ascompared with light emitting diodes (LED) and laser diodes (LD).

The EL devices emit light under the influence of an electric field. Thefunction of a semiconductor layer constituting such an EL device isbased on the radiative recombination of electron-hole pairs injectedfrom a pair of electrodes into the semiconductor. Exemplary devices arelight emitting diodes (LED) based on GaP and similar Group III-Group Vsemiconductors. Although these devices are effectively and widelyutilized, their size is so small that it is not only difficult, but alsouneconomical to apply the LEDs to large area displays. Several types ofmaterials are known as the substitutes applicable to large areadisplays. Of these inorganic semiconductors, ZnS is most useful. The ZnSsystem, however, suffers from serious practical drawbacks including thelack of reliability. One exemplary mechanism associated with ZnS isdeemed to be that carriers of one type are accelerated through thesemiconductor under a strong electric field, to induce the localexcitation of the semiconductor which is relaxed by radiative lightemission.

It is known that of organic materials, simple aromatic molecules such asanthracene, perylene, and coronene are electroluminescent.

On practical use, these organic materials have the problems that theylack reliability like ZnS, and joining layers of these materials tocurrent-injecting electrode layers is difficult.

The technique of depositing organic materials through sublimation leavesthe problem that the resulting layers are soft and likely torecrystallize.

The technique of building up properly modified aromatic compounds byLangmuir-Blodgett method invites deterioration of film quality, dilutionof active material, and an increase of manufacturing cost.

An EL device using anthracene is disclosed in U.S. Pat. No. 3,621,321.This device has the inconveniences of increased power consumption andlow luminescence.

Another attempt to provide an improved device is U.S. Pat. No. 4,672,265which discloses an EL device comprising a luminescent layer of a doublelayer structure.

However, the materials used in the double layer structure are organicmaterials having the above-mentioned inconveniences.

JP-A 10-92576 discloses an EL device comprising a semiconductor layer inthe form of a thin dense polymer film composed of at least oneconjugated polymer, a first contact layer adjoining a first surface ofthe semiconductor layer, and a second contact layer adjoining a secondsurface of the semiconductor layer. The polymer film of which thesemiconductor layer is formed has a sufficiently low concentration ofexternal charge carriers so that, when an electric field is appliedbetween the first and second contact layers across the semiconductorlayer with the second contact layer made positive relative to the firstcontact layer, charge carriers are injected into the semiconductor layerwhereby the semiconductor layer emits light.

Conjugated polymers themselves are known, and their application tooptical modulators, for example, is disclosed in European PatentApplication No. 0294061. Polyacetylene is used as an active layer in amodulating structure between first and second electrodes. An insulatinglayer must be disposed between one electrode and the active layer so asto form a space charge region in the active layer providing an opticalmodulation effect. Nevertheless, the presence of the space charge regiondisables formation of electron-hole pairs that emit light through theirdecay. Therefore, such a structure fails to exert electroluminescence.The development of electroluminescence is utterly undesirable inEuropean Patent Application No. 0294061 because the optical modulationeffect is destroyed thereby.

To solve these problems, it is contemplated to utilize the advantages ofboth an organic material and an inorganic semiconductor material.Specifically, an organic/inorganic semiconductor junction is given bysubstituting an organic p-type semiconductor for the organic holetransporting layer. Such studies are disclosed in Japanese Patent No.2636341, JP-A 2-139893, 2-207488, and 6-119973. However, it is difficultto design devices which surpass conventional organic EL devices withrespect to luminescent performance and device reliability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an organic EL devicecapable of utilizing the advantages of both organic and inorganicmaterials and having a high efficiency, long lifetime, and low cost.

This and other objects are achieved by the present invention which isdefined below.

(1) An organic electroluminescent device comprising a substrate, a holeinjecting electrode and an electron injecting electrode formed on thesubstrate, and an organic material-containing organic layer between theelectrodes,

the organic layer including a light emitting layer containing aconjugated polymer,

the device further comprising an inorganic insulative electron injectingand transporting layer between the light emitting layer and the electroninjecting electrode,

the inorganic insulative electron injecting and transporting layercomprising

at least one oxide selected from the group consisting of lithium oxide,rubidium oxide, potassium oxide, sodium oxide, and cesium oxide as afirst component,

at least one oxide selected from the group consisting of strontiumoxide, magnesium oxide, and calcium oxide as a second component, and

silicon oxide, germanium oxide or a mixture of silicon oxide andgermanium oxide as a third component.

(2) The organic electroluminescent device of (1) wherein the inorganicinsulative electron injecting and transporting layer contains 5 to 95mol % of the first component, 5 to 95 mol % of the second component, and5 to 95 mol % of the third component, based on the entire components.

(3) The organic electroluminescent device of (1) wherein the inorganicinsulative electron injecting and transporting layer has a thickness of0.1 to 2 nm.

(4) The organic electroluminescent device of (1) wherein the electroninjecting electrode is formed of at least one metal element selectedfrom the group consisting of Al, Ag, In, Ti, Cu, Au, Mo, W, Pt, Pd, andNi.

(5) The organic electroluminescent device of (1) further comprising aninorganic insulative hole injecting and transporting layer between thelight emitting layer and the hole injecting electrode,

the inorganic insulative hole injecting layer comprising silicon oxideor germanium oxide or a mixture of silicon oxide and germanium oxide asa main component,

the main component having an average composition represented by theformula: (Si_(1−x)Ge_(x))O_(y) wherein x is from 0 to 1 and y is from1.7 to 1.99, as analyzed by Rutherford back-scattering.

(6) The organic electroluminescent device of (5) wherein the inorganicinsulative hole injecting and transporting layer has a thickness of 0.1to 3 nm.

FUNCTION

In the organic EL device of the invention, the conjugated polymer usedin the light emitting layer is preferably poly(p-phenylene vinylene).The polymer film preferably has a generally uniform thickness in therange of 10 nm to 5 μm. The conjugated polymer preferably has asemiconductor band gap in the range of 1 eV to 3.5 eV. Also preferably,the proportion of the conjugated polymer in an electroluminescent zonein the polymer film is sufficient to achieve the percolation thresholdto ensure charge transfer within the conjugated polymer in the film.

The conjugated polymer designates a polymer having a nonlocalized πelectron system along its main skeleton. The nonlocalized π electronsystem endows the polymer with semiconductive properties as well as anability to carry positive and negative charge carriers having a highmobility along the polymer skeleton.

Such polymers are discussed by R. H. Friend in Journal of MolecularElectronics, 4 (1988), January-March, No. 1, pp. 37-46, for example.

In the organic EL device, the hole injecting electrode and holeinjecting layer serve to inject positive charge carriers into thepolymer film whereas the electron injecting electrode and electroninjecting layer serve to inject negative charge carriers into thepolymer film. These charge carriers are combined to form charge pairssusceptible to radiative decay. For this reason, it is preferred thatthe hole and electron injecting electrodes be selected so as to havehigh and low work functions, respectively.

To acquire the desired electroluminescence, the polymer film ispreferably substantially free of defects which act as the center ofnon-luminescent recombination. Such defects obstruct theelectroluminescence.

In addition to the charge injecting function, the inorganic insulativehole injecting and transporting layer or electron injecting andtransporting layer, at least one of which is formed, serves to controlthe ratio of electrons to holes injected into the electroluminescentlayer and to ensure that radiative decay occurs apart from the chargeinjecting material of the layer in contact therewith.

The film of conjugated polymer is preferably composed of a singleconjugated polymer or a single copolymer comprising segments of aconjugated polymer. Alternatively, the film of conjugated polymer may becomposed of a mixture of a conjugated polymer or copolymer and anothersuitable polymer.

Further preferred characteristics of the polymer film are given below.(i) The polymer is stable upon exposure to oxygen, humidity and hightemperature. (ii) The polymer film has good adhesion to the underlyinglayer, an ability to inhibit the occurrence of cracks caused bytemperature rise and pressure bias, and resistance to shrinkage,expansion, recrystallization or other morphological changes. (iii) Owingto high crystallinity and a high melting point, for example, the polymerfilm is recoverable with respect to an ion/atom transfer step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of an organic EL device according to a first embodiment ofthe invention.

FIG. 2 is a schematic cross-sectional view illustrating theconfiguration of an organic EL device according to a second embodimentof the invention.

FIG. 3 is a schematic cross-sectional view illustrating theconfiguration of a prior art organic EL device.

FIG. 4 is a schematic cross-sectional view illustrating theconfiguration of another prior art organic EL device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The organic EL device of the invention includes a pair of a holeinjecting electrode and an electron injecting electrode, and an organiclayer participating in a light emitting function disposed between theelectrodes, the organic layer including a light emitting layercomprising a conjugated polymer. The device further includes aninorganic insulative electron injecting and transporting layer disposedbetween the light emitting layer and the electron injecting electrode.

When combined with the inorganic electron injecting and transportinglayer to be described later, the negative electrode is not criticalbecause it need not have a low work function and an electron injectingability. Conventional metals may be used. Inter alia, one or more metalelements selected from among Al, Ag, In, Ti, Cu, Au, Mo, W, Pt, Pd andNi, especially Al and Ag are preferable for conductivity and ease ofhandling.

The negative electrode thin film may have at least a sufficientthickness to supply electrons to the inorganic electron injecting andtransporting layer, for example, a thickness of at least 50 nm,preferably at least 100 nm. Although the upper limit is not critical,the film thickness is typically about 50 to about 500 nm.

In the organic EL device of the invention, when combined with theinorganic electron injecting and transporting layer to be describedlater, the negative electrode preferably uses the above-described metalsalthough the following materials may also be used if necessary.Exemplary materials are, for example, single metal elements such as K,Li, Na, Mg, La, Ce, Ca, Sr, Ba, Sn, Zn, and Zr, binary or ternary alloysconsisting of two or three of these elements for improving stability,such as Ag—Mg (Ag: 0.1 to 50 at %), Al—Li (Li: 0.01 to 14 at %), In—Mg(Mg: 50 to 80 at %), and Al—Ca (Ca: 0.01 to 20 at %).

The electron injecting electrode thin film may have a sufficientthickness to effect electron injection, for example, a thickness of atleast 0.1 nm, preferably at least 0.5 nm, more preferably at least 1 nm.Although the upper limit is not critical, the film thickness istypically about 1 to about 500 nm. On the electron injecting electrode,an auxiliary or protective electrode may be provided, if desired.

The auxiliary electrode may have at least a sufficient thickness toensure efficient electron injection and prevent the ingress of moisture,oxygen and organic solvents, for example, a thickness of at least 50 nm,preferably at least 100 nm, more preferably 100 to 500 nm. A too thinauxiliary electrode layer would exert its effect little, lose a stepcoverage capability, and provide insufficient connection to a terminalelectrode. If too thick, greater stresses are generated in the auxiliaryelectrode layer, accelerating the growth rate of dark spots.

For the auxiliary electrode, an appropriate material may be chosen inconsideration of the material of the electron injecting electrode to becombined therewith. For example, low resistivity metals such as aluminummay be used when electron injection efficiency is of importance. Metalcompounds such as TiN may be used when sealing is of importance.

The thickness of the electron injecting electrode and the auxiliaryelectrode combined is usually about 50 to about 500 nm though it is notcritical.

The hole injecting electrode is preferably formed of materials capableof effectively injecting holes into the hole injecting layer, morepreferably materials having a work function of 4.5 to 5.5 eV.Illustratively, materials based on tin-doped indium oxide (ITO),zinc-doped indium oxide (IZO), indium oxide (In₂O₃), tin oxide (SnO₂) orzinc oxide (ZnO) are preferable. These oxides may deviate somewhat fromtheir stoichiometry. An appropriate proportion of SnO₂ mixed with In₂O₃is about 1 to 20%, more preferably about 5 to 12% by weight. For IZO, anappropriate proportion of ZnO mixed with In₂O₃ is about 12 to 32% byweight.

The hole injecting electrode may contain silicon oxide (SiO₂) foradjusting the work function. When added to ITO, the content of siliconoxide is preferably about 0.5 to 10 mol % of ITO. The inclusion ofsilicon oxide increases the work function of ITO.

The electrode on the light exit side should preferably have a lighttransmittance of at least 80%, especially at least 90% in the lightemission band, typically from 400 to 700 nm, and especially at eachlight emission. With a lower transmittance, the light emitted by thelight emitting layer would be attenuated through the electrode, failingto provide a luminance necessary as a light emitting device.

Preferably the electrode has a thickness of 50 to 500 nm, especially 50to 300 nm. Although the upper limit of the electrode thickness is notcritical, a too thick electrode would cause a drop of transmittance andseparation. Too thin an electrode would be insufficient for its effectand low in film strength during fabrication.

The light emitting layer contains a conjugated polymer. The conjugatedpolymer used in the light emitting layer is preferably poly(p-phenylenevinylene), abbreviated as PPV, of the following formula (I).

In the formula, the phenylene ring may have one or more substituentsindependently selected from among alkyl (preferably methyl), alkoxy(preferably methoxy or ethoxy), halogen (preferably chloro or bromo),and nitro, if desired.

Other conjugated polymers derived from poly(p-phenylene vinylene) arealso appropriate as the conjugated polymer used herein.

Typical examples of these derivatives are shown below.

(i) Polymers of the structure represented by formulae (II) to (IV): Theyare obtained by substituting a fused ring for the phenylene ring informula (I), for example, by substituting an anthracene or naphthalenering for the phenylene ring.

These polycyclic systems may also have one or more substituents asdescribed in conjunction with the phenylene ring.

(ii) Polymers of the structure represented by formula (V): They areobtained by substituting a heterocyclic ring such as a furan ring forthe phenylene ring.

The furan ring may also have one or more substituents as described inconjunction with the phenylene ring.

(iii) Polymers of the structure represented by formulae (VI) to (VIII):They are obtained by increasing the number of vinylene moieties attachedto respective phenylene rings or other rings described above in (i) or(ii).

In the above structural formulae, y is equal to 2, 3, 4, 5, 6 or 7.Usually, n is from about 3 to about 10,000.

These rings may also have one or more substituents as described inconjunction with the phenylene ring.

These distinct PPV derivatives have different semiconductor energy gaps.By properly selecting and mixing PPVs having different semiconductorenergy gaps, it becomes possible to construct an EL device which emitslight at different wavelengths over the entire visible spectrum.

The conjugated polymer film can be prepared by chemically treatingand/or heat treating a polymer “precursor” which is solution or meltprocessable. The polymer precursor can be purified or pretreated into adesired shape before it is subsequently converted into a conjugatedpolymer through elimination reaction.

Films of the above-described PPV derivatives may be similarly formed onorganic EL structures using suitable sulfonium precursors.

It is sometimes advantageous to use a polymer precursor having a highersolubility in an organic solvent than a sulfonium salt precursor (II).The solubility of the precursor in an organic solvent can be increasedby substituting a less hydrophilic group such as an alkoxy group(typically methoxy) or pyridinium group for the sulfonium moiety in theprecursor.

Typically, on a substrate having formed thereon an electrode andoptionally a hole injecting layer, electron injecting layer or the like,a film of poly(phenylene vinylene) can be formed by a method based onthe following reaction scheme.

In an aqueous solution, water/ethanol mixture or methanol, the sulfoniumsalt monomer (II) is converted into a polymer precursor (III). Thesolution of prepolymer (III) can be applied onto a substrate by theconventional spin coating technique commonly used for photoresistprocessing in the semiconductor industry. Otherwise, a coating may beformed by casting, dipping, bar coating, roll coating and othertechniques. The thus obtained polymer precursor (III) film is thenconverted into poly(phenylene vinylene) (I), typically by heating at atemperature of 200 to 350° C.

With respect to the conditions required for the chemical synthesis ofmonomer (II), polymerization of monomer (II) into precursor (III), andthermal conversion of precursor (III) into PPV (I), reference should bemade to the literature, for example, D. D. C. Bradley, J. Phys. D(Applied Physics), 20, 1389 (1987) and J. D. Stenger Smith, R. W. Lenzand G. Wegner, Polymer, 30, 1048 (1989).

The film of poly(phenylene vinylene) preferably has a thickness of 0.1nm to 10 μm, more preferably 0.5 nm to 1 μm, most preferably 10 to 500nm. The PPV film has only few pinholes. The PPV film has a semiconductorenergy gap of about 2.5 eV (500 nm). The PPV film is tough,substantially inert to oxygen at room temperature, and stable to gasesother than air at temperatures in excess of 300° C.

By modifying the leaving group on the polymer precursor to ensure thatelimination reaction proceeds as single reaction without forming anotherintermediate structure, the ordering of the material can be improved.Therefore, for example, the n-dialkylsulfonium component can be replacedby a tetrahydrothiophenium component. The latter component is eliminatedas a single leaving group without being decomposed into alkyl mercaptanas is the dialkylsulfide. The polymer precursor used in the exampledescribed herein encompasses those in which dimethylsulfide andtetratryebrothiophene are selected as the dialkylsulfonium component.These precursors form PPV films appropriate for use in organic ELdevices.

Additionally, the preferred material of which the conjugated polymerfilm is formed is poly(phenylene).

This material can be prepared by starting with derivatives which arebiochemically synthesized from 5,6-dihydroxycyclohexa-1,3-diene. Usingradical initiators, these derivatives can be polymerized into polymerprecursors which are soluble in a single solvent. The preparation ofpoly(phenylene) is described in Ballard et al, J. Chem. Comm., 954(1983).

A solution of the polymer precursor is spin coated onto a substrate as athin film, which is converted into conjugated poly(phenylene) polymer byheat treating typically at a temperature of 140 to 240° C.

In forming a phenylene copolymer, copolymerization may be similarlyeffected using a vinyl or diene monomer.

Other preferred types of material that can be used in forming conjugatedpolymer films include conjugated polymers which themselves are solutionprocessable or melt processable due to the presence of a giantside-chain group attached to a primary conjugated chain or byincorporating a conjugated polymer into a copolymer structure in whichone or more components are non-conjugated. Examples of the formerconjugated polymers are given below.

(a) Poly(4,4′-diphenylene diphenyl vinylene), abbreviated as PDPV, is anarylene vinylene polymer in which the carbons in both vinylene moietiesare substituted with phenyl rings. Since this polymer is soluble inordinary organic solvents, a thin film can be formed therefrom.

(b) Poly(1,4-phenylene-1-phenylvinylene) and poly(1,4-phenylene diphenylvinylene) are analogous to PPV. They are polymers in which the carbon orcarbons in one or both vinylene moieties are substituted with phenylgroups. They are soluble in organic solvents and can be cast or spincoated to form thin films.

(c) Poly(3-alkylthiophene) polymers wherein the alkyl is selected fromamong propyl, butyl, pentyl, hexyl, heptyl, octyl, decyl, undecyl,dodecyl, etc. are solution processable in ordinary organic solvents, andmelt processable depending on the long alkyl sequence (alkyl is equal toor longer than octyl).

(d) Poly(3-alkylpyrrole) polymers are expected to be analogous to thepoly(3-alkylthiophene) polymers.

(e) Poly(2,5-dialkoxy-p-phenylene vinylene) polymers in which the alkylis longer than butyl are solution processable.

(f) Poly(phenylacetylene) polymers are polyacetylene derivatives inwhich a hydrogen atom on the backbone is replaced by a phenyl group.This replacement renders the polymers soluble.

In order to ensure processability necessary for polymers and facilitateformation of a uniform thin film onto a substrate (having an electrodeand necessary functional thin films formed thereon), it is sometimessuitable to form a polymer blend of a conjugated polymer and anotherpolymer.

When such a copolymer or a polymer blend is used in forming a conjugatedpolymer film, the active sites in the EL device having the conjugatedpolymer film incorporated therein must contain a number of conjugatedpolymer sites which are equal to or greater than the percolationthreshold of the copolymer or polymer blend.

The light emitting layer is formed as a composite layer includingpolymer layers having different band gaps and/or majority chargespecies, which achieves the concentration of electric charges injectedfrom the hole/electron injecting layers into the light emitting layer ora specific region within the light emitting layer. The composite layercan be formed by continuous precipitation of polymer layers. When adifferent film is applied to a conjugated polymer in the form of aprecursor by spin or draw coating, that film is made insoluble by theconversion step to conjugated polymer, the subsequent layer can besimilarly applied without dissolving the previously applied film.

Further, the following polymers requiring no heat polymerization stepmay be used as the conjugated polymer in the light emitting layer.

Useful conjugated polymers are soluble in solvents, have a numberaverage molecular weight of 10³ to 10⁷, have a structure of continuousconjugated bonds, have recurring units of at least two different types,each recurring unit having at least one conjugated bond, and form a thinfilm exhibiting such properties that the difference between the peakwavelength of its absorption spectrum and the peak wavelength of itsfluorescent spectrum is at least 120 nm. More preferred are conjugatedpolymers containing 0.01 to 40 mol % of the recurring units thatrespectively form homopolymers whose optical absorption edge energy isminimum. It is noted that the number average molecular weight isdetermined by gel permeation chromatography (GPC) using chloroform asthe solvent and calculated on the basis of polystyrene.

From the standpoint of providing conjugated polymer fluorescentmaterials having a high fluorescence quantum yield, the conjugatedpolymer fluorescent materials are preferably conjugated polymerscomprising recurring structures represented by the following formulae(1) to (3). More preferred are conjugated polymers comprising recurringstructures having alternately joined vinylene and aryl or heterocyclicgroups, represented by the following formulae (4) and (5).

The preferred recurring units of the conjugated polymers used herein aredivalent aromatic compound groups or derivative groups and divalentheterocyclic compound groups or derivative groups shown below.

Herein, R₁ to R₅₇ are independently hydrogen, alkyl, alkoxy andalkylthio groups of 1 to 20 carbon atoms, aryl and aryloxy groups of 6to 18 carbon atoms, or heterocyclic compound groups of 4 to 14 carbonatoms.

Exemplary are a combination of a divalent aromatic compound group orderivative group with a vinylene group, and a combination of a divalentheterocyclic compound group or derivative group with a vinylene group,as shown by the following formulae (1) to (5).

—Ar1-CH═CH—  (1)

—Ar2-CH═CH—  (2)

—Ar3-CH═CH—  (3)

Herein, Ar1, Ar2, and Ar3, which are different from each other, arearylene groups or divalent heterocyclic compound groups each forming aconjugated bond continuous to the vinylene group, and at least one ofAr1, Ar2 and Ar3 is an arylene or heterocyclic compound group having atleast one substituent selected from the class consisting of alkyl,alkoxy or alkylthio groups of 4 to 22 carbon atoms, aryl or aryloxygroups of 6 to 60 carbon atoms, and heterocyclic compound groups of 4 to60 carbon atoms.

—Ar4-CH═CH—Ar5-CH═CH—  (4)

—Ar5-CH═CH—Ar6-CH═CH—  (5)

Herein, Ar4, Ar5, and Ar6, which are different from each other, arearylene groups or divalent heterocyclic compound groups each forming aconjugated bond continuous to the vinylene group, and at least one ofAr4, Ar5 and Ar6 is an arylene or heterocyclic compound group having atleast one substituent selected from the class consisting of alkyl,alkoxy or alkylthio groups of 4 to 22 carbon atoms, aryl or aryloxygroups of 6 to 60 carbon atoms, and heterocyclic compound groups of 4 to60 carbon atoms.

Preferred among these groups are phenylene, substituted phenylene,biphenylene, substituted biphenylene, naphthalene diyl, substitutednaphthalene diyl, anthracene-9,10-diyl, substitutedanthracene-9,10-diyl, pyridine-2,5-diyl, substituted pyridine-2,5-diyl,thienylene, and substituted thienylene groups. Phenylene, biphenylene,naphthalene diyl, pyridine-2,5-diyl, and thienylene groups are morepreferred.

With respect to the substituent, examples of the alkyl group having 1 to20 carbon atoms include methyl, ethyl, propyl, butyl, pentyl, hexyl,heptyl, octyl, decyl and lauryl, with the methyl, ethyl, pentyl, hexyl,heptyl and octyl being preferred. Examples of the alkoxy group having 1to 20 carbon atoms include methoxy, ethoxy, propoxy, butoxy, pentyloxy,hexyloxy, heptyloxy, octyloxy, decyloxy, and lauryloxy, with themethoxy, ethoxy, pentyloxy, hexyloxy, heptyloxy, and octyloxy beingpreferred. Examples of the alkylthio group include methylthio,ethylthio, propylthio, butylthio, pentylthio, hexylthio, heptylthio,octylthio, decylthio, and laurylthio, with the methylthio, ethylthio,pentylthio, hexylthio, heptylthio, and octylthio being preferred.Examples of the aryl group include phenyl, 4-(C₁-C₁₂)alkoxyphenyl,4-(C₁-C₁₂)alkylphenyl, 1-naphthyl, and 2-naphthyl. Phenoxy is a typicalaryloxy group. Exemplary of the heterocyclic compound group are2-thienyl, 2-pyrrolyl, 2-furyl, and 2-, 3- or 4-pyridyl.

When conjugated polymers comprising recurring units selected from theabove-mentioned units form a thin film exhibiting such properties thatthe difference between the peak wavelength of its absorption spectrumand the peak wavelength of its fluorescent spectrum is at least 120 nm,they are luminescent materials having a high fluorescence quantum yield.

Of the conjugated polymers comprising recurring units selected from theabove-mentioned units, those copolymers containing 0.01 to 40 mol % ofthe recurring units whose optical absorption edge energy is minimum arepreferred. From these recurring units, a choice is preferably made ofthose recurring units which respectively form homopolymers between whichthe difference in optical absorption edge energy is at least 0.05 eV,because they form luminescent materials having a very high fluorescencequantum yield. To this end, at least two different types of chemicalstructures must be selected.

Further preferably, Ar1, Ar2 and Ar3 are selected from differentchemical structures. The recurring units between which the difference inoptical absorption edge energy is at least 0.05 eV are exemplified bythose in which Ar1, Ar2 and Ar3 have substituents, at least one of whichis an alkoxy group, alkylthio group, aryloxy group or heterocycliccompound group of at least 4 carbon atoms, or one or two of Ar1, Ar2 andAr3 are selected from heterocyclic compound groups.

With respect to the recurring units containing Ar4, Ar5, and Ar6, thoserecurring units in which Ar4, Ar5, and Ar6 are different from eachother, and Ar4, Ar5, and Ar6 have substituents, at least one of which isan alkoxy group, alkylthio group, aryloxy group or heterocyclic compoundgroup of at least 4 carbon atoms, or one of Ar4 and Ar6 is aheterocyclic compound group provide conjugated polymers having a highfluorescence yield.

It is noted that the conjugated polymers may be random, block or graftcopolymers, or polymers having an intermediate structure, for example,random copolymers with partial block structures. From the standpoint ofobtaining copolymers having a high fluorescence quantum yield, randomcopolymers with partial block structures and block or graft copolymersare preferred to fully random copolymers.

The preferred solvents for the polymeric fluorescent materials accordingto the invention are chloroform, methylene chloride, dichloroethane,tetrahydrofuran, toluene, xylene, etc. The polymeric fluorescentmaterials may be dissolved in these solvents in amounts of at least 0.1%by weight although the solubility depends on the structure and molecularweight of polymeric fluorescent materials. In order to obtain polymershaving good film-forming abilities such as solvent solubility, it ispreferred that in the combination of Ar1, Ar2 and Ar3 or the combinationof Ar4, Ar5 and Ar6, at least one is an aryl or heterocyclic compoundgroup which is nuclearly substituted with at least one substituentselected from the class consisting of alkyl, alkoxy and alkylthio groupsof 4 to 22 carbon atoms, aryl and aryloxy groups of 6 to 60 carbonatoms, and heterocyclic compound groups of 4 to 60 carbon atoms.

These substituents are exemplified below. Examples of the alkyl grouphaving 4 to 22 carbon atoms include butyl, pentyl, hexyl, heptyl, octyl,decyl and lauryl, with the pentyl, hexyl, heptyl and octyl beingpreferred. Examples of the alkoxy group having 4 to 22 carbon atomsinclude butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy, decyloxy, andlauryloxy, with the pentyloxy, hexyloxy, heptyloxy, and octyloxy beingpreferred. Examples of the alkylthio group include butylthio,pentylthio, hexylthio, heptylthio, octylthio, decylthio, and laurylthio,with the pentylthio, hexylthio, heptylthio, and octylthio beingpreferred. Examples of the aryl group include phenyl,4-(C₁-C₁₂)alkoxyphenyl, 4-(C₁-C₁₂)alkylphenyl, 1-naphthyl, and2-naphthyl. Phenoxy is a typical aryloxy group. Exemplary of theheterocyclic compound group are 2-thienyl, 2-pyrrolyl, 2-furyl, and 2-,3- or 4-pyridyl.

From these recurring units, a choice is preferably made of thoserecurring units which respectively form homopolymers between which thedifference in optical absorption edge energy is at least 0.05 eV. Also,from the standpoint of obtaining copolymers with a high solubility, thecontent in the polymer of the recurring units having these substituentsshould preferably be 5 to 100 mol %, more preferably 15 to 100 mol %.

The degree of polymerization of the inventive polymer is not criticaland varies with the type and proportion of recurring structures. Fromthe film-forming standpoint, the total number of recurring structures ispreferably from about 3 to 10,000, more preferably from about 3 to3,000, most preferably from about 4 to 2,000.

In fabricating organic EL devices, when films are formed from solutionsusing these polymers soluble in organic solvents, it is simply requiredto apply the solution and remove the solvent by drying. Even when thepolymer is mixed with a charge transporting material to be describedlater, a similar procedure is applicable. This is very advantageous inthe manufacture.

Typical of the inventive copolymers are arylene vinylene copolymers.Their synthesis method is not critical. For example, copolymers can beformed using a method similar to the methods described in JP-A 1-254734and 1-79217. More particularly, one exemplary method is a hydrogenhalide-removing method of copolymerizing two or more correspondingbis(methyl halide) compounds, for example, 2,5-diethyl-p-xylylenedibromide, 2,5-diheptyloxy-p-xylylene dibromide and p-xylylene dibromidein a xylene/tert-butyl alcohol mixture in the presence oftert-butoxypotassium. Random copolymers are generally formed by thismethod although block copolymers can be formed using oligomers.

Also useful is Witting reaction in which corresponding bis(methylhalide) compounds, for example, 2,5-diethyl-p-xylylene dibromide and2,5-diheptyloxy-p-xylylene dibromide are reacted with triphenylphosphinein N,N-dimethylformamide solvent to synthesize a phosphonium salt, whichis polymerized with a corresponding dialdehyde compound, for example,terephthalaldehyde in ethyl alcohol in the presence of lithium ethoxide.To form a copolymer, two or more diphosphonium salts and/or two or moredialdehyde compounds may be reacted. Another exemplary process is asulfonium salt-decomposing process in which a corresponding sulfoniumsalt is polymerized in the presence of alkali, followed by sulfoniumsalt-removing treatment. When these polymers are used as the luminescentmaterial in organic EL devices, they are preferably purified aftersynthesis as by re-precipitation or chromatographic fractionationbecause luminescent characteristics are governed by their purity.

The structure of the organic EL device which is fabricated using theluminescent material according to the invention is not critical insofaras the luminescent material of the above-mentioned polymer is used inthe light emitting layer disposed between a pair of electrodes at leastone of which is transparent or translucent. Any of well-known structuresmay be employed. Exemplary is a structure wherein a light emitting layerof the polymeric fluorescent material mentioned above or a mixture ofthe polymeric fluorescent material and a charge transporting material(which is used to encompass both an electron transporting material and ahole transporting material) is sandwiched between a pair of electrodes.Also useful is a structure wherein an electron transporting layercomprising an electron transporting material is interleaved between thelight emitting layer and the electron injecting electrode and/or a holetransporting layer comprising a hole transporting material isinterleaved between the light emitting layer and the hole injectingelectrode.

It is also encompassed within the scope of the invention that each ofthe light emitting layer and the charge transporting layer is either asingle layer or a combination of two or more layers. Further, in thelight emitting layer, a luminescent material other than the polymericfluorescent material may be used in admixture. A layer in which thepolymeric fluorescent material and/or charge transporting material isdispersed in a polymer may also be used.

The charge transporting materials used along with the polymer accordingto the invention, that is, electron transporting materials and holetransporting materials may be selected from well-known ones and are notparticularly limited. Exemplary hole transporting materials includepyrazoline derivatives, arylamine derivatives, stilbene derivatives, andtriphenyldiamine derivatives. Exemplary electron transporting materialsinclude oxadiazole derivatives, anthraquinodimethane and derivativesthereof, benzoquinone and derivatives thereof, naphthoquinone andderivatives thereof, anthraquinone and derivatives thereof,tetracyanoanthraquinodimethane and derivatives thereof, fluorenonederivatives, diphenyldicyanoethylene and derivatives thereof,diphenoquinone derivatives, and metal complexes of 8-hydroxyquinolineand derivatives thereof.

More illustratively, useful charge transporting materials are described,for example, in JP-A 63-70257, 63-175860, 2-135359, 2-135361, 2-209988,3-37992, and 3-152184. Preferred hole transporting materials aretriphenyldiamine derivatives. Preferred electron transporting materialsinclude oxadiazole derivatives, benzoquinone and derivatives thereof,anthraquinone and derivatives thereof, and metal complexes of8-hydroxyquinoline and derivatives thereof. The especially preferredhole transporting material is4,4-bis(N-(3-methylphenyl)-N-phenylamino)biphenyl. The especiallypreferred electron transporting materials are2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole, benzoquinone,anthraquinone, and tris(8-quinolinol)aluminum. Among these compounds,either one or both of the electron transporting compound and the holetransporting compound may be used. They may be used alone or inadmixture of two or more.

When a charge injecting layer of organic material is disposed between alight emitting layer and an electrode, any of these charge transportingmaterials may be used to form an organic charge injecting layer. In casea charge transporting material is used in admixture with a luminescentmaterial in the light emitting layer, the amount of the chargetransporting material used varies with the type and other factors of aparticular compound used and may be determined, with such factors takeninto account, so as to fall within the range that ensures a film-formingability and does not impair luminescent characteristics. Usually, theamount of the charge transporting material used is 1 to 40% by weight,more preferably 2 to 30% by weight based on the luminescent material.

The luminescent material which can be used with the polymericfluorescent material according to the invention may be selected fromwell-known luminescent materials, for example, naphthalene derivatives,anthracene and derivatives thereof, perylene and derivatives thereof,polymethine dyes, xanthene dyes, coumarin dyes, and cyanine dyes, metalcomplexes of 8-hydroxyquinoline and derivatives thereof, aromaticamines, tetraphenylcyclopentadiene and derivatives thereof,tetraphenylbutadiene and derivatives thereof. Illustratively, a choicemay be made of the materials described in JP-A 57-51781 and 59-194393,for example.

On the electrode is formed a light emitting layer comprising theabove-mentioned polymer as a luminescent material or the polymer and acharge transporting material. The light emitting layer is formed bycoating processes such as spin coating, casting, dipping, bar coating,and roll coating processes, using solutions, mix solutions or melts ofthe above materials. Preferably the layer is formed by coating processessuch as spin coating, casting, dipping, bar coating, and roll coatingprocesses, using solutions or mix solutions.

Where the conjugated polymer fluorescent material is used, the lightemitting layer has a thickness of 0.5 nm to 10 μm, preferably 1 nm to 1μm. In order to increase the current density for improving the luminousefficiency, the light emitting layer preferably has a thickness of 10 to500 nm. When a thin film is to be formed by the coating process, thecoating is desirably heat dried in vacuum or in an inert atmosphere at atemperature of 30 to 200° C., more preferably 60 to 100° C., forremoving the solvent. When such a heat drying step is necessary, it ispreferred to form an inorganic charge injecting layer, to be describedbelow, between the light emitting layer and the electrode.

The organic EL device of the invention has at least an inorganicinsulative electron injecting and transporting layer and preferably bothan inorganic insulative electron injecting and transporting layer and aninorganic insulative hole injecting and transporting layer as theinorganic insulative charge injecting layer between the light emittinglayer and a pair of electrodes. The provision of these charge injectinglayers ensures efficient injection of electric charges into the lightemitting layer to produce the desired luminescence.

The inorganic insulative electron injecting and transporting layer iscomposed of three components: first, second and third components (thethird component being also designated a stabilizer). This eliminates aneed to form a special electrode having an electron injecting functionand permits a metal electrode having relatively high stability and goodconductivity to be used. And the electron injecting and transportingefficiency of the inorganic insulative electron injecting andtransporting layer is improved, and the lifetime of the device isprolonged.

The inorganic insulative electron injecting and transporting layercontains at least one oxide selected from the group consisting oflithium oxide (Li₂O), rubidium oxide (Rb₂O), potassium oxide (K₂O),sodium oxide (Na₂O), nd cesium oxide (Cs₂O), as the first component.These oxides may be used alone or in admixture of two or more. Themixture of two or more oxides may have an arbitrary mix ratio. Of theseoxides, lithium oxide (Li₂O) is most preferred, while rubidium oxide(Rb₂O), potassium oxide (K₂O) and sodium oxide (Na₂O) are next preferredin the described order. When these oxides are used in admixture, themixture should preferably contain at least 40 mol %, and especially atleast 50 mol % of lithium oxide and rubidium oxide combined.

The inorganic insulative electron injecting and transporting layercontains at least one oxide selected from the group consisting ofstrontium oxide (SrO), magnesium oxide (MgO), and calcium oxide (CaO) asthe second component. These oxides may be used alone or in admixture oftwo or more. The mixture of two or more oxides may have an arbitrary mixratio. Of these oxides, strontium oxide (SrO) is most preferred,magnesium oxide (MgO) is second best, and calcium oxide (CaO) is thirdbest. When these oxides are used in admixture, the mixture shouldpreferably contain at least 40 mol % of strontium oxide.

The inorganic insulative electron injecting and transporting layercontains silicon oxide (SiO₂) and/or germanium oxide (GeO₂) as the thirdcomponent or stabilizer. Either one or both of silicon oxide andgermanium oxide may be used. The mixture of silicon oxide and germaniumoxide may have an arbitrary mix ratio.

These oxides are generally present in stoichiometric composition, butmay deviate more or less therefrom.

Also preferably, the inorganic insulative electron injecting andtransporting layer according to the invention contains the respectivecomponents in the following amounts:

first component: 5 to 95 mol %, more preferably 50 to 90 mol %,

second component: 5 to 95 mol %, more preferably 50 to 90 mol %, and

third component: 0.5 to 20 mol %, more preferably 5 to 10 mol %,

based on the entire components, provided that the respective componentsare calculated as SrO, MgO, CaO, Li₂O, Rb₂O, K₂O, Na₂O, Cs₂O, SiO₂, andGeO₂.

The thickness of the inorganic insulative electron injecting andtransporting layer is not critical although it is preferably about 0.1to 2 nm, especially about 0.3 to 0.8 nm thick.

The inorganic insulative electron injecting and transporting layer maybe formed by various physical and chemical thin-film forming processessuch as sputtering and electron beam (EB) vapor deposition, with thesputtering process being preferred.

When the inorganic insulative electron injecting layer is formed bysputtering, the sputtering gas is preferably under a pressure of 0.1 to1 Pa during sputtering. The sputtering gas used may be an inert gascommonly used in conventional sputtering apparatus, such as Ar, Ne, Xeor Kr. If necessary, N₂ may be used. The sputtering atmosphere may bethe sputtering gas in admixture with 1 to 99% of O₂. The target used isthe above-mentioned oxide while either single source sputtering ormulti-source sputtering is acceptable. Often, the target is a mix targetcontaining the main component, auxiliary component and additives. A filmdeposited therefrom has a composition substantially equivalent to orslightly oxygen poorer than that of the target.

The sputtering process used may be a high-frequency sputtering processusing an RF power supply or DC sputtering process, with the RFsputtering being preferred. The power to the sputtering apparatus ispreferably in the range of 0.1 to 10 W/cm² for RF sputtering while thedeposition rate is preferably 0.1 to 50 nm/min, especially 1 to 10nm/min.

It is noted that when the inorganic electron injecting and transportinglayer is deposited, the organic layer or the like can be subjected toashing and hence, damaged. Under such a situation, it is recommendedthat the inorganic electron injecting layer is deposited as two layers.That is, the layer is initially thinly deposited in the absence ofoxygen and then thickly in the presence of oxygen. The thickness reachedin the absence of oxygen is preferably about ⅕ to about ⅘ of the overallthickness. The oxygen poor layer which is deposited in the absence ofoxygen is preferably adjusted to about 60 to 90% of the normal oxygencontent. The oxidized layer which is deposited in the presence of oxygenis available in the stoichiometric composition of normal oxide althoughit may deviate somewhat from the stoichiometry. Accordingly, thedifference of oxygen content between the oxygen poor layer and theoxidized layer is preferably at least 10%, especially at least 20%.Also, the oxygen content may continuously vary within the above-definedrange.

The temperature of the substrate during deposition is from roomtemperature (25° C.) to about 150° C.

The inorganic insulative hole injecting and transporting layer which isprovided in the preferred embodiment is composed of an oxide of siliconand/or germanium as a main component.

The main component has an average composition represented by theformula:

(Si_(1−x)Ge_(x))O_(y)

wherein 0≦x≦1 and 1.7≦y≦1.99, as analyzed by Rutherford back-scattering.

By controlling the oxide as the main component of the inorganicinsulative hole injecting and transporting layer so as to fall in theabove compositional range, holes can be efficiently injected from thehole injecting electrode into the organic layer on the light emittinglayer side. Additionally, migration of electrons from the organic layerto the hole injecting electrode is restrained, and effectiverecombination of holes and electrons in the light emitting layer isensured. Since hole injection and transportation is intended, no lightemission occurs with a reverse bias voltage applied. The organic ELdevice of the invention has both the advantages of inorganic materialand the advantages of organic material and is effectively applicable todisplays of the time-division drive mode which are required to produce ahigh light emission luminance. The organic EL device of the inventionproduces a luminance comparable to that of prior art devices having anorganic hole injecting layer. Owing to high heat resistance and weatherresistance, the organic EL device of the invention has a longer servicelife than the prior art devices and develops minimal leaks and darkspots. The use of an inexpensive, readily available inorganic materialrather than relatively expensive organic materials offers the advantagesof easy manufacture and a reduced manufacture cost.

Letter y representative of the oxygen content is from 1.7 to 1.99. If yis outside this range, the layer has a reduced hole injectingcapability, leading to a drop of luminance. Preferably y is from 1.85 to1.98.

The inorganic insulative hole injecting and transporting layer may be athin film consisting essentially of silicon oxide, germanium oxide, or amixture of silicon oxide and germanium oxide. Letter x representative ofthe germanium to silicon ratio is from 0 to 1. Preferably x is up to0.4, more preferably up to 0.3, especially up to 0.2.

Alternatively, x is preferably at least 0.6, more preferably at least0.7, especially at least 0.8.

The oxygen content is given as an average value for the film as analyzedby Rutherford back-scattering.

In addition to the oxide, the hole injecting layer may contain up to 10at % in total of impurities such as Ne, Ar, Kr and Xe used as thesputtering gas. Preferably the hole injecting layer contains about 0.01to 2%, more preferably 0.05 to 1.5% by weight of Ne, Ar, Kr and Xe.These impurity elements may be contained alone or in admixture of two ormore. The mixture may be of two or more impurity elements in anarbitrary ratio.

These elements are used as the sputtering gas and thus introduced intothe inorganic insulative hole injecting and transporting layer duringits formation. If the content of impurity elements is too high, thelayer would lose the trapping capability and hence the desiredperformance.

The amount of sputtering gas used in film formation is determined by thepressure, the flow rate ratio of sputtering gas to oxygen, depositionrate, and other factors, and especially the pressure. In order tocontrol the content of sputtering gas in the deposited film so as tofall within the above-described range, it is preferred to effect filmdeposition in higher vacuum, specifically in a vacuum of 1 Pa or lower,especially 0.1 to 1 Pa.

As long as the overall hole injecting layer has the above-describedcomposition on the average, the composition of the hole injecting layerneed not be uniform. A layer structure having a graded concentration ina thickness direction is acceptable. In this case, the hole injectinglayer is preferably oxygen poorer on the interface with the organiclayer (or light emitting layer).

The inorganic insulative hole injecting and transporting layer isnormally amorphous.

The thickness of the inorganic insulative hole injecting layer is notcritical although an appropriate thickness is about 0.05 nm to about 10nm, more preferably about 0.1 nm to about 5 nm, especially about 1 toabout 5 nm or about 0.5 to about 3 nm. Hole injection would becomeinsufficient when the thickness of the hole injecting layer is outsidethe range.

Methods for preparing the inorganic insulative hole injecting layerinclude various physical and chemical thin film forming methods such assputtering and electron beam (EB) vapor deposition, with the sputteringbeing preferred.

When the inorganic insulative hole injecting layer is formed bysputtering, the sputtering gas is preferably under a pressure of 0.1 to1 Pa during sputtering. The sputtering gas may be any of inert gasesused in conventional sputtering equipment, for example, Ar, Ne, Xe andKr. Nitrogen (N₂) gas may be used if necessary. Reactive sputtering maybe carried out in an atmosphere of the sputtering gas mixed with 1 to99% of oxygen (O₂) gas. The target used herein is the above-describedoxide or oxides, and either single source or multiple source sputteringmay be carried out.

The sputtering process may be an RF sputtering process using an RF powersource or a DC reactive sputtering process, with the former beingpreferred. The power of the sputtering equipment is preferably in therange of about 0.1 to about 10 W/cm² for RF sputtering. The depositionrate is preferably in the range of about 0.5 to about 10 nm/min.,especially about 1 to about 5 nm/min. The substrate is kept at roomtemperature (25° C.) to about 150° C. during deposition.

Reactive sputtering is acceptable. When nitrogen is incorporated, thereactive gas may be N₂, NH₃, NO, NO₂, N₂O, etc. When carbon isincorporated, the reactive gas may be CH₄, C₂H₂, CO, etc. These reactivegases may be used alone or in admixture of two or more.

By providing the inorganic hole injecting layer and the inorganicelectron injecting layer, the organic EL device of the invention isimproved in heat resistance and weather resistance so that a longerlifetime is expectable. The use of inexpensive, readily availableinorganic materials rather than relatively expensive organic materialsoffers the advantages of easy manufacture and a reduced manufacturecost. Furthermore, the invention improves the connection to electrodesof inorganic materials which was a problem in the prior art. Thisminimizes the occurrence of leakage current and the development of darkspots.

In the organic EL device of the invention, a hole injecting andtransporting layer of organic material may be provided instead of theinorganic hole injecting and transporting layer. In this embodiment,when a heat polymerization step is required in forming a light emittinglayer, the layer that underlies the light emitting layer shouldpreferably be an inorganic electron injecting and transporting layer,because heating can reach about 300° C. on the high temperature side.

For the hole injecting and transporting layer of organic material, it ispreferable to use the following hole injecting and transportingmaterials.

The hole injecting and transporting compound is preferably selected fromamine derivatives having strong fluorescence, for example,triphenyldiamine derivatives which are hole transporting materials,styrylamine derivatives and amine derivatives having an aromatic fusedring.

As the hole injecting and transporting compound, there may be usedvarious organic compounds as described, for example, in JP-A 63-295695,2-191694, 3-792, 5-234681, 5-239455, 5-299174, 7-126225, 7-126226, and8-100172, and EP 0650955A1. Exemplary are tetraarylbenzidine compounds(triaryldiamines or triphenyldiamines: TPD), aromatic tertiary amines,hydrazone derivatives, carbazole derivatives, triazole derivatives,imidazole derivatives, oxadiazole derivatives having an amino group, andpolythiophenes. These compounds may be used alone or in admixture of twoor more. When two or more of these compounds are used, they may beformed as separate layers or mixed.

When the hole injecting layer is formed below the light emitting layerwhose formation requires a heat polymerization step, the hole injectinglayer must be resistant to heat. Preferred in this embodiment are holeinjecting and transporting compounds having a glass transitiontemperature of at least 200° C., more preferably at least 300° C., mostpreferably at least 350° C.

The thicknesses of the organic hole injecting layer and the organicelectron injecting layer are not critical and vary with a particularformation technique. Usually, their thickness is preferred to range fromabout 5 nm to about 500 nm, especially about 10 nm to about 300 nm. Whenthe hole or electron injecting and transporting layer is divided into aninjecting layer and a transporting layer, preferably the injecting layeris at least 1 nm thick and the transporting layer is at least 1 nmthick. The upper limit of thickness is usually about 500 nm for theinjecting layer and about 500 nm for the transporting layer.

In forming the organic hole injecting and transporting layer and theorganic electron injecting layer, vacuum evaporation is preferably usedbecause homogeneous thin films are available. By utilizing vacuumevaporation, there is obtained a homogeneous thin film which isamorphous or has a crystal grain size of less than 0.2 μm. If the grainsize is more than 0.2 μm, uneven light emission would take place and thedrive voltage of the device must be increased with a substantial drop ofhole or electron injection efficiency.

The conditions for vacuum evaporation are not critical although a vacuumof 10⁻⁴ Pa or lower and a deposition rate of about 0.01 to 1 nm/sec arepreferred. It is preferred to successively form layers in vacuum becausethe successive formation in vacuum can avoid adsorption of impurities onthe interface between the layers, thus ensuring better performance.Also, the drive voltage of a device can be reduced and the developmentand growth of dark spots be restrained.

In the embodiment wherein the respective layers are formed by vacuumevaporation, where it is desired for a single layer to contain two ormore compounds, boats having the compounds received therein areindividually temperature controlled to achieve co-deposition.

Further preferably, a shield plate may be provided on the device inorder to prevent the organic layers and electrodes from oxidation. Inorder to prevent the ingress of moisture, the shield plate is attachedto the substrate through an adhesive resin layer for sealing. Thesealing or filler gas is preferably an inert gas such as argon, helium,and nitrogen. The filler gas should preferably have a moisture contentof less than 100 ppm, more preferably less than 10 ppm, especially lessthan 1 ppm. The lower limit of the moisture content is usually about 0.1ppm though not critical.

The shield plate is selected from plates of transparent or translucentmaterials such as glass, quartz and resins, with glass being especiallypreferred. Alkali glass is preferred because of economy although otherglass compositions such as soda lime glass, lead alkali glass,borosilicate glass, aluminosilicate glass, and silica glass are alsouseful. Of these, plates of soda glass without surface treatment areinexpensive and useful. Metal plates and plastic plates may also be usedas the shield plate.

Using a spacer for height adjustment, the shield plate may be held at adesired height over the layer structure. The spacer may be formed fromresin beads, silica beads, glass beads, and glass fibers, with the glassbeads being especially preferred. Usually the spacer is formed fromparticles having a narrow particle size distribution while the shape ofparticles is not critical. Particles of any shape which does notobstruct the spacer function may be used. Preferred particles have anequivalent circle diameter of about 1 to 20 μm, more preferably about 1to 10 μm, most preferably about 2 to 8 μm. Particles of such diametershould preferably have a length of less than about 100 μm. The lowerlimit of length is not critical although it is usually equal to or morethan the diameter.

When a shield plate having a recess is used, the spacer may be used ornot. When used, the spacer should preferably have a diameter in theabove-described range, especially 2 to 8 μm.

The spacer may be premixed in a sealing adhesive or mixed with a sealingadhesive at the time of bonding. The content of the spacer in thesealing adhesive is preferably 0.01 to 30% by weight, more preferably0.1 to 5% by weight.

Any of adhesives which can maintain stable bond strength and gastightness may be used although UV-curable epoxy resin adhesives ofcation curing type are preferred.

The substrate on which the organic EL structure is formed may beselected from amorphous substrates of glass and quartz and crystallinesubstrates of Si, GaAs, ZnSe, ZnS, GaP, and InP, for example. Ifdesired, buffer layers of crystalline materials, amorphous materials ormetals may be formed on such crystalline substrates. Metal substratesincluding Mo, Al, Pt, Ir, Au and Pd are also useful. Of these, glasssubstrates are preferred. Since the substrate is often situated on thelight exit side, the substrate should preferably have a lighttransmittance as described above for the electrode.

A plurality of inventive devices may be arrayed on a plane. A colordisplay is obtained when the respective devices of a planar array differin emission color.

The substrate may be provided with a color filter film, a fluorescentmaterial-containing color conversion film or a dielectric reflectingfilm for controlling the color of light emission.

The color filter film used herein may be a color filter as used inliquid crystal displays and the like. The properties of a color filtermay be adjusted in accordance with the light emission of the organic ELdevice so as to optimize the extraction efficiency and color purity.

It is also preferred to use a color filter capable of cutting externallight of short wavelength which is otherwise absorbed by the EL devicematerials and fluorescence conversion layer, because the lightresistance and display contrast of the device are improved.

An optical thin film such as a multilayer dielectric film may be usedinstead of the color filter.

The fluorescence conversion filter film is to convert the color of lightemission by absorbing electroluminescence and allowing the fluorescentmaterial in the film to emit light. It is formed from three components:a binder, a fluorescent material, and a light absorbing material.

The fluorescent material used may basically have a high fluorescencequantum yield and desirably exhibits strong absorption in the ELwavelength region. In practice, laser dyes are appropriate. Use may bemade of rhodamine compounds, perylene compounds, cyanine compounds,phthalocyanine compounds (including sub-phthalocyanines), naphthalimidecompounds, fused ring hydrocarbon compounds, fused heterocycliccompounds, styryl compounds, and coumarin compounds.

The binder is selected from materials which do not cause extinction offluorescence, preferably those materials which can be finely patternedby photolithography or printing technique. Also, where the filter filmis formed on the substrate so as to be contiguous to the hole injectingelectrode, those materials which are not damaged during deposition ofthe hole injecting electrode (such as ITO or IZO) are preferable.

The light absorbing material is used when the light absorption of thefluorescent material is short and may be omitted if unnecessary. Thelight absorbing material may also be selected from materials which donot cause extinction of fluorescence of the fluorescent material.

The organic EL device of the invention is generally of the dc or pulsedrive type while it can be of the ac drive type. The applied voltage isgenerally about 2 to 30 volts.

As shown in FIG. 1, the organic EL device of the invention may have thesuccessively stacked configuration of substrate 1/hole injectingelectrode 2/light emitting layer 4/inorganic insulative electroninjecting layer 5/negative electrode (or electron injecting electrode)6. As shown in FIG. 2, the device may have the successively stackedconfiguration of substrate 1/hole injecting electrode 2/inorganicinsulative hole injecting and transporting layer 3/light emitting layer4/inorganic insulative electron injecting layer 5/negative electrode (orelectron injecting electrode) 6. The inorganic insulative hole injectinglayer may be replaced by a hole injecting layer of organic material. Theorder of layer stacking may be reversed, that is, a reversely stackedconfiguration is acceptable. Among these configurations, a choice may beproperly made in accordance with the specifications of the desireddisplay and manufacturing process. In the configurations of FIGS. 1 and2, a drive power supply E is connected between the hole injectingelectrode 2 and the negative electrode (or electron injecting electrode)6.

The device of the invention may have a multi-stage configuration ofelectrode layer/inorganic layer (inorganic insulative hole injectinglayer or inorganic insulative electron injecting layer) and lightemitting layer/electrode layer/inorganic layer and light emittinglayer/electrode layer/inorganic layer and light emitting layer/electrodelayer, or further repeated layers. Such a multi-stage configuration iseffective for adjusting or multiplying the color of emitted light.

The organic EL device of the invention is applicable as displays,optical pickups for use in writing and reading of memories, repeaters intransmission lines for optical communication, photocouplers, and otheroptical devices.

EXAMPLE Example 1

A substrate of (7059) glass by Corning Glass Works was scrubbed using aneutral detergent.

By RF magnetron sputtering from a target of ITO oxide, a hole injectingelectrode layer of ITO having a thickness of 200 nm was formed on thesubstrate at a temperature of 250° C.

After its ITO electrode-bearing surface was cleaned with UV/O₃, thesubstrate was secured by a holder for spin coating.

A methanol solution of a PPV precursor having a concentration of 1 g ofpolymer per 10 to 25 g of methanol was spin coated onto the substratehaving the inorganic insulative hole injecting layer deposited thereon.This coating was carried out by applying the polymer solution to theentire surface of the substrate and then rotating the substrate at arevolution of up to 5,000 rpm while keeping its upper surfacehorizontal.

The substrate with the polymer precursor layer was heated for 12 hoursin a vacuum oven at a temperature of 300° C. This heat treatmentconverted the polymer precursor into PPV. The resulting PPV film was 100to 300 nm thick.

The substrate was transferred to a sputtering chamber, which wasevacuated to a vacuum of 1×10⁻⁴ Pa or lower.

Using a target obtained by mixing raw materials: strontium oxide (SrO),lithium oxide (Li₂O), and silicon oxide (SiO₂) so that the targetconsisted of

80 mol % of Sro,

10 mol % of Li₂O,

and

10 mol % of SiO₂,

based on the entire components, an inorganic electron injecting andtransporting layer was deposited to a thickness of 0.8 nm. Depositingconditions included a substrate temperature of 25° C., sputtering gasAr, a deposition rate of 1 nm/min, an operating pressure of 0.5 Pa, andan input power of 5 W/cm². The inorganic electron injecting andtransporting layer was deposited to a thickness of 0.4 nm whilesupplying 100 SCCM of 100% Ar as the sputtering gas, and then to athickness of 0.4 nm while supplying 100 SCCM of a mixture of Ar/O₂(1/1).

With the vacuum kept, Al was evaporated to a thickness of 200 nm to forma negative electrode. A glass shield was sealed to the substrate toenclose the layer structure therein, completing an organic EL device. Acomparative sample was fabricated in which the negative electrode wasformed on the light emitting layer without forming the inorganicelectron injecting layer.

An electric field was applied across the organic EL device in air, whichexhibited diode characteristics. When biased to be positive on the ITOside and negative on the Al side, the current flow increased as thevoltage increased. A distinct light emission was observed in an ordinaryroom.

The organic EL devices were driven at a constant current density of 10mA/cm². The comparative sample exhibited only a luminance of 100 cd/m²whereas the inventive sample produced a luminance of 500 cd/m². Theluminance half-life was improved by a factor of more than 5 over thecomparative sample.

Example 2

In Example 1, the main component, auxiliary component and stabilizer ofthe inorganic insulative electron injecting and transporting layer werechanged from SrO to MgO, CaO or mixtures of these oxides, from Li₂O toK₂O, Rb₂O, K₂O, Na₂O, Cs₂O or mixtures of these oxides, and from SiO₂ toGeO₂ or oxide mixtures of SiO₂ and GeO₂, respectively, withsubstantially equivalent results. Similar results were obtained when thenegative electrode forming material was changed from Al to Ag, In, Ti,Cu, Au, Mo, W, Pt, Pd, Ni or alloys thereof.

Example 3

In Example 1, after its ITO electrode-bearing surface was cleaned withUV/O₃, the substrate was secured by a holder in a vacuum evaporationchamber, which was evacuated to a vacuum of 1×10⁻⁴ Pa or lower.

Using a target of SiO₂, an inorganic insulative hole injecting layer wasdeposited to a thickness of 2 nm. The sputtering gas used herein was amixture of Ar and 5% of O₂. Depositing conditions included a substratetemperature of 25° C., a deposition rate of 1 nm/min, an operatingpressure of 0.5 Pa, and an input power of 5 W/cm². The hole injectinglayer thus deposited had a composition of SiO_(1.9).

Thereafter, a PPV film was formed as in Example 1, and an inorganicelectron injecting layer, an Al—Li (Li: 7 at %) film, and an Al filmwere formed, obtaining an organic EL device.

The resulting organic EL device was evaluated as in Example 1, findingsubstantially equivalent results.

Example 4

Organic EL devices were fabricated as in Example 1 except that in thestep of depositing the inorganic insulative hole injecting andtransporting layer in Examples 1 to 3, the flow rate of O₂ in thesputtering gas was changed and the target used was changed in accordancewith the desired layer composition so that the resulting layers had thecompositions SiO_(1.7), SiO_(1.95), GeO_(1.96), andSiO_(0.5)Ge_(0.5)O_(1.92), respectively. The devices were tested foremission luminance and life, obtaining substantially equivalent results.

Example 5

Synthesis of polymeric fluorescer

A phosphonium salt (A) was synthesized by reacting2,5-diethyl-p-xylylene dibromide with triphenylphosphine inN,N-dimethylformamide solvent. Also, a phosphonium salt (B) wassynthesized by reacting 2,5-diheptyloxy-p-xylylene dibromide withtriphenylphosphine in N,N-dimethylformamide solvent. In ethyl alcoholwere dissolved 4.1 parts by weight of phosphonium salt (A), 1.0 part byweight of phosphonium salt (B), and 0.8 part by weight ofterephthalaldehyde. An ethyl alcohol solution containing 0.8 part byweight of lithium ethoxide was added dropwise to the ethyl alcoholsolution of phosphonium salts and dialdehyde. Polymerization waseffected for 3 hours at room temperature. The reaction mixture wasallowed to stand overnight at room temperature. The precipitate wascollected by filtration, washed with ethyl alcohol, and dissolved inchloroform, to which ethanol was added for re-precipitation. Theprecipitate was dried in vacuum, obtaining 0.35 part by weight of apolymer. This is designated Polymeric Fluorescer 1. The recurring unitsof Polymeric Fluorescer 1 are shown below together with their molarratio as calculated from the ratio of monomer charges.

This Polymeric Fluorescer 1 has a number average molecular weight of5.0×10³ calculated on the basis of polystyrene. With respect to thestructure of Polymeric Fluorescer 1, absorption peaks at 960 cm⁻¹ forvinylene, 1520 cm⁻¹ for phenylene, 1100 cm⁻¹ for ether, and 2860 cm⁻¹for alkyl were found in its infrared absorption spectrum. On ¹H-NMRanalysis using chloroform-d as a solvent, H in phenylene vinylene group(near 6.5 to 8.0 ppm), H of —OCH₂— in heptyloxy group (near 3.5 to 4.0ppm), and H of —CH₂— in ethyl group (near 2.5 ppm) were found. The molarratio of recurring units as calculated from their intensity ratio wassubstantially coincident with the value calculated from the ratio ofmonomer charges.

Organic EL devices were fabricated as in Examples 1 to 4, except that a1.0 wt % chloroform solution of Polymeric Fluorescer 1 was used in theformation of the PPV film. The solution was applied to a thickness of 50nm by a dipping technique, followed by vacuum drying at 80° C. for onehour, forming the light emitting layer.

The organic EL device was examined as in Example 1, finding equivalentresults to Examples 1 to 4.

BENEFITS

There has been described an organic EL device which can take advantageof both organic and inorganic materials, and has an improved efficiency,an extended effective life, and a low cost.

What is claimed is:
 1. An organic electroluminescent device comprising asubstrate, a hole injecting electrode and an electron injectingelectrode formed on the substrate, and an organic material-containingorganic layer between the electrodes, said organic layer including alight emitting layer containing a conjugated polymer, said devicefurther comprising an inorganic insulative electron injecting andtransporting layer between said light emitting layer and said electroninjecting electrode, said inorganic insulative electron injecting andtransporting layer comprising at least one oxide selected from the groupconsisting of lithium oxide, rubidium oxide, potassium oxide, sodiumoxide, and cesium oxide as a first component, at least one oxideselected from the group consisting of strontium oxide, magnesium oxide,and calcium oxide as a second component, and silicon oxide, germaniumoxide or a mixture of silicon oxide and germanium oxide as a thirdcomponent.
 2. The organic electroluminescent device of claim 1 whereinsaid inorganic insulative electron injecting and transporting layercontains 5 to 95 mol % of the first component, 5 to 95 mol % of thesecond component, and 5 to 95 mol % of the third component, based on theentire components.
 3. The organic electroluminescent device of claim 1wherein said inorganic insulative electron injecting and transportinglayer has a thickness of 0.1 to 2 nm.
 4. The organic electroluminescentdevice of claim 1 wherein said electron injecting electrode is formed ofat least one metal element selected from the group consisting of Al, Ag,In, Ti, Cu, Au, Mo, W, Pt, Pd, and Ni.
 5. The organic electroluminescentdevice of claim 1 further comprising an inorganic insulative holeinjecting and transporting layer between said light emitting layer andsaid hole injecting electrode, said inorganic insulative hole injectinglayer comprising silicon oxide or germanium oxide or a mixture ofsilicon oxide and germanium oxide as a main component, the maincomponent having an average composition represented by the formula:(Si_(1−x)Ge_(x))O_(y) wherein x is from 0 to 1 and y is from 1.7 to1.99, as analyzed by Rutherford back-scattering.
 6. The organicelectroluminescent device of claim 5 wherein said inorganic insulativehole injecting and transporting layer has a thickness of 0.1 to 3 nm. 7.The organic electroluminescent device of claim 1, wherein the conjugatedpolymer is poly(p-phenylene vinylene), optionally substituted on thephenylene ring with a group selected from alkyl, alkoxy, halogen, andnitro.
 8. The organic electroluminescent device of claim 1, wherein theconjugated polymer is present in the form of a film having a thicknessof 10 nm to 5 μm.
 9. The organic electroluminescent device of claim 1,wherein the electron injecting electrode has a thickness of 0.1 to 500nm.
 10. The organic electroluminescent device of claim 1, furthercomprising an auxiliary electrode disposed on the electron injectingelectrode.
 11. The organic electroluminescent device of claim 10,wherein the auxiliary electrode has a thickness of 50 to 500 nm.
 12. Theorganic electroluminescent device of claim 1, wherein the hole injectingelectrode comprises a material selected from the group consisting ofITO, IZO, In₂O, SnO₂, and ZnO.
 13. The organic electroluminescent deviceof claim 12, wherein the hole injecting electrode further comprisesSiO₂.
 14. The organic electroluminescent device of claim 1, wherein thehole injecting electrode has a light transmittance of at least 80%. 15.The organic electroluminescent device of claim 1, wherein the holeinjecting electrode has a thickness of 50 to 500 nm.
 16. The organicelectroluminescent device of claim 1, wherein the conjugated polymer isselected from the group consisting of polymers having the followingstructures (II) to (VIII):


17. The organic electroluminescent device of claim 2, wherein theinorganic insulative electron injecting and transporting layer comprises50 to 90 mol % of the first component.
 18. The organicelectroluminescent device of claim 2, wherein the inorganic insulativeelectron injecting and transporting layer comprises 50 to 90 mol % ofthe second component.
 19. The organic electroluminescent device of claim2, wherein the inorganic insulative electron injecting and transportinglayer comprises 5 to 10 mol % of the third component.
 20. The organicelectroluminescent device of claim 5, wherein y is from 1.85 to 1.98.21. The organic electroluminescent device of claim 5, wherein x is from0 to 0.2.
 22. The organic electroluminescent device of claim 5, whereinx is from 0.8 to
 1. 23. The organic electroluminescent device of claim5, wherein the inorganic hole injecting and transporting layer furthercomprises 0.05 to 1.5% by weight of at least one element selected fromthe group consisting of Ne, Ar, Kr, and Xe.
 24. The organicelectroluminescent device of claim 1, further comprising an organic holeinjecting and transporting layer disposed between the light emittinglayer and the hole injecting electrode, wherein the organic holeinjecting and transporting layer comprises at least one organic compoundselected from the group consisting of a triaryldiamine, atriphenyldiamine, an aromatic tertiary amine, a hydrazone, a carbazole,a triazole, an imidazole, an oxadiazole having an amino group, and apolythiophene.
 25. The organic electroluminescent device of claim 1,further comprising a shield plate attached to the substrate by means ofan adhesive.